It is well known that contact lenses are becoming more and more popular in our society. Many people are wearing contact lenses as opposed to conventional eyeglasses for reasons of convenience, improved appearance, lighter weight, and correction of sight abnormalities over a broad visual field. Most conventional contact lenses are made from methyl methacrylate. Lenses made from this material are known as "hard lenses". These lenses suffer from many deficiencies. For example, such lenses frequently produce corneal edema and/or a condition of extreme discomfort to the wearer's eye after repeated periods of extended wear, i.e., eighteen hours or more. This situation is known to be due to "oxygen starvation" and may also be associated with inadequate dissipation of carbon dioxide.
The epithelium of the cornea requires oxygen which is usually supplied from the oxygen dissolved in tears. However, because of the manner in which lenses conform to the contour of the eye, the flow of lacrimal fluid is greatly curtailed beneath the lens. This reduction in fresh lacrimal fluid is not desirable as it substantially reduces the contact of the eye with oxygen. Therefore, it is extremely important that the lens material itself be gas permeable. Prior art lenses have been of a material and thickness which fails to admit sufficient oxygen and/or release sufficient carbon dioxide to maintain a healthy normal condition for the eye tissue and cornea covered, especially when the lens is worn continuously for extended periods of time. In other words, the conventional lens cannot breathe through the body of the lens satisfactorily.
Due to the above problems, many workers in the field have experimented with the production of soft contact lenses. The presently known soft contact lenses are made of hydrophilic polymers, mainly comprising polyhydroxyethyl methacrylate (known in the art as "HEMA"). These hydrogel soft lenses are an improvement over the hard lenses but the materials themselves are not gas permeable. However, these materials absorb water and swell until equilibrium is attained and therefore possess a high degree of hydration which is directly related to the mode of oxygen transport. The highly hydrated lenses are able to obtain satisfactory oxygen transport levels but suffer from several resulting problems. First, since the soft lenses are used in the swollen state, the molecular materials of their composition are markedly reduced in mechanical strength and are extremely fragile. Due to this fragileness, the thickness of the lens must be increased and therefore these prior art soft lenses are ill-suited for the preparation of ultra-thin corneal lenses. By increasing the thickness of the lens, the gas permeability of the lens is thereby decreased forming a vicious cycle between gas permeability and strength.
In making an ultra-thin lens, the greater the strength and the greater the refractive index of the material used, the better the resulting thin lens.
A second problem associated with the prior art soft lenses is that since they are always worn in the wet and swollen state, they are easily contaminated with bacteria. Therefore, they need to be sterilized once a day by boiling. This boiling treatment is not only troublesome, but often causes decomposition and breakdown of the lens material. Thus, the prior art soft lenses are very short lived.
With specific reference to prior art presently considered to be most closely related to the subject invention, Hamm, U.S. Pat. No. 3,072,622, discloses polymerization products of alkyl styrenes with alkyl methacrylates suitable for use with as polymers for, e.g., lenses.
Lang et al, U.S. Pat. No. 3,080,348 discloses copolymers of styrene and methyl methacrylate, prepared by particular processing conditions.
Kaetsu et al, U.S. Pat. No. 3,983,083 teach soft contact lens polymers formed by the copolymerization of hydroxy alkyl acrylate or methacrylate in which the hydroxy alkyl contains two or three atoms, with another hydroxy alkyl acrylate or methacrylate in which the alkyl contains from four to 13 carbon atoms. Kaetsu et al discuss the problems for which their invention is one solution as follows:
"The presently known soft contact lenses are made of hydrophilic polymers . . . . These materials absorb water and swell until equilibrium is obtained and in the wet swollen state they are soft and flexible. Therefore, the known soft contact lenses are characterized in that they are always used in the swollen state. In such a swollen state, however, high molecular weight materials generally are markedly reduced in mechanical strength and are extremely fragile. In addition, since they are worn always in the wet and swollen state, these soft contact lenses are easily contaminated with bacteria. Therefore they need to be sterilized once a day by boiling. The boiling treatment is not only troublesome but often causes decomposition and breakdown of the lens material. Thus, the prior art soft contact lenses are very short-lived because of their characteristics of being used in the wet and swollen state. This is a serious practical defect and prevents their wide acceptance. Also, the prior art soft contact lenses are difficult to manufacture. Although the prior art soft contact lenses are used in the swollen state, the work of shaping and finishing such as cutting, machining, polishing, etc., must be applied to the polymeric material in the dry state, but it is very difficult to tell beforehand the precise shape, size, curvature and the like which the lens will assume in the swollen state. . . . In short, the known process for manufacturing the prior art soft contact lenses is complicated and difficult to control and the percentage of rejection is very high." PA1 "The thus obtained polymeric material is soft, flexible per se and can be used as is as soft contact lenses without swelling treatments. However, being soft in itself, this material is difficult to precisely cut, machine and polish and, therefore, cannot directly be subjected to shaping and finishing procedures to make soft contact lenses having precisely the desired shape, size and curvature. One aspect of the present invention (of Kaetsu et al) is characterized in that the thus obtained polymeric material for soft contact lenses is cooled nearly to the glass transition temperature of the polymer not higher than 0.degree. C. to make it rigid and then, in the rigid state, is cut, machined and polished to produce a contact lens with the precise configuration." PA1 "The thus obtained copolymer was gradually cooled with liquid nitrogen and was subjected to machining to impart the configuration and size as a contact lens . . . ." PA1 (a) a group 1 monomer which is a styrene or substituted styrene monomer, and PA1 (b) a group 2 monomer which is a monomer of the general formula: ##STR3## wherein R is hydrogen, methyl or ethyl and R' is an alkyl, or a monomer of the general formula: ##STR4## wherein n is 0 or 1, R is hydrogen, methyl or ethyl, and R" is hydrogen when n is 0 and hydrogen or hydroxyl when n is 1. PA1 1. A biocompatible lens material with excellent tissue compatability; PA1 2. A chemically inert lens material which will resist hydrolysis even on boiling or immersion in alcohol; PA1 3. A fine, very clear optical lens material with excellent refraction (e.g., styrene at about 1.56 with acrylate at 1.45); PA1 4. A lens material of surprisingly good gas permeability since styrene materials, contrary to the conventional methyl methacrylate, enhance gas permeability; PA1 5. A lens material which can tolerate a very high degree of hydration and yet retain its strength further enhancing gas permeability; PA1 6. Good lens strength and toughness despite relatively high hydration and thin cross-section, e.g., a lens with 48% hydration and only about half a millimeter thick was found to have serviceable strength during handling while with prior art lenses, such thickness would render the lens too fragile; PA1 7. A lens of superior mechanical integrity (even as an ultra-thin lens) having dimensional stability when boiled, molded, or machined while still being elastic and highly hydrated as a soft lens. PA1 8. Surprising "graft-susceptibility", e.g., for grafting a bifocal section into a conventional soft lens; PA1 9. A lens material that is surprisingly practical and inexpensive, styrene being commonly available at low cost.
Kaetsu et al then go on to disclose their invention in which monomers of Group I and II, the two hydroxy alkyl monomer classes, are copolymerized sometimes with the addition of a Group III polymer to add strength. The polymerization is carried out using the conventional polymerization cross-linkers, initiators and techniques.
Kaetsu et al describe the product of their process, i.e., their soft lens polymer as follows:
Kaetsu et al give a number of examples of their polymers of which Example 6 is probably worthy of specific consideration. In Example 6, 20 parts of hydroxy propyl methacrylate and 40 parts of heptane diol monomethacrylate and 10 parts of triethylene glycol dimethacrylate and 10 parts of styrene are copolymerized together. The description of Example 6 then continues:
Kaetsu et al, thus, teaches the copolymerization of two different hydroxy alkyl acrylates or methacrylates. Kaetsu et al also teaches that if the polymers are formed in the way they are suggested, these polymers must be chilled to near the glass transition temperature, below 0.degree. C., to be machined into lenses.
The disclosed invention obviates the above deficiencies in the prior art by providing a copolymer suitable for producing contact lenses which have a superiorly high strength and refractive index, that can withstand sterilization, and, in addition, offer superior gas permeability. These properties make the fabrication of an ultra-thin lens a practical reality.